In electric vehicles and hybrid vehicles, it is a common practice to use an electric motor which is connected to the drive axle of the vehicle as a generator that produces electric power and provides a braking force at the same time when decelerating the vehicle. This is called as regenerative braking. Such a brake-by-wire vehicle brake system that combines the hydraulic braking and the regenerative braking may also be incorporated with an ABS (anti-lock brake system) and a VSA (vehicle stability assist) system as disclosed in JP 2009-227023. In such a vehicle brake system, an ABS hydraulic unit and a VSA hydraulic unit are combined with a motor actuated cylinder which is typically provided with a simulator for producing a brake pedal reaction.
A brake-by-wire system is typically provided with a conventional master cylinder which is directly connected to the brake pedal. In case of a failure in the brake system, the hydraulic pressure produced from the master cylinder is forwarded to the wheel cylinders. However, in normal operation, the motor actuated cylinder provides the necessary brake fluid pressure which is actuated according to the stroke of the brake pedal detected by a stroke sensor while the output line of the master cylinder is shut off by a solenoid shut-off valve. This solenoid shut-off valve typically consists of a normally open solenoid valve which shuts every time the brake pedal is operated.
A normally open solenoid valve is normally held open by a return spring, and is closed by energizing the solenoid that displaces a plunger in a valve closing direction against the spring force of the return spring. The fully open position and fully closed position of the solenoid shut-off valve is defined by the plunger being engaged by corresponding mechanical stoppers. It is known that some impact noise is inevitable every time the plunger reaches the fully open position and the fully closed position under the spring force and the magnetic force, respectively.
In particular, in a brake-by-wire brake system, the solenoid valve in the output line of the master cylinder closes and opens every time the brake pedal is depressed and released so that any slight noise caused by the operation of the solenoid valve could create a problem. The noises may be reduced by reducing the electric power supplied to the solenoid valve, but it impairs the responsiveness of the solenoid valve, and is therefore not acceptable as a solution to the problem in most applications.